The invention relates to a variable valve actuation mechanism for an internal combustion engine, an internal combustion engine comprising a variable valve actuation mechanism, and a vehicle comprising such an engine.
In internal combustion engines for vehicles, e.g. light vehicles such as personal cars, or heavy vehicles, such as trucks, it is known to have systems for changing the characteristics for the actuations of the intake and/or exhaust valves, e.g. the timing and/or the degree of opening of the valves.
Various techniques are known for such variable valve actuation (VVA) systems. For example, one of them is cam switching, in which adjustment mechanisms are provided in the cam followers. Cam switching concepts may include followers in the form of switchable levers, in which some parts are movable in relation to other parts.
US2012325168 relates to a switchable lever for a cam shifting system. The lever comprises two rolls, one of which is movable for coming into and out of contact with one of two cam lobes. US2011265750 and US2011265751 also relate to switchable levers for cam shifting systems, with rolls movable between positions of a high-lift cam contact and a low-lift cam contact.
Another VVA technique is known as the concentric camshaft concept. Therein, the adjustment mechanisms are provided in the camshaft arrangement, the follower parts are fixed in relation to each other. The concentric camshaft concept involves coaxial camshafts and combined cam lobe profiles. For the valve, or the valves, for the intake or exhaust function at each cylinder, one follower spans a pair of closely spaced cam lobes. Two camshafts are arranged in a concentric manner. The cam lobes are fixed to a respective of the camshafts, and can thereby, by twisting of one camshaft in relation to the other, be moved in relation to each other so as the change the combined profile of the two lobes.
Known solution with the concentric camshaft concept are disclosed in U.S. Pat. Nos. 1,527,456A, 4,771,742A and 8,820,281. US2015007789 discloses a valve gear with two camshafts and two vane rotors coupled to a respective of the camshafts.
There is a desire to reduce wear in variable valve actuation mechanisms, which are subject to harsh conditions with long durations and a very high number of cycles.
It is desirable to reduce wear in variable valve actuation mechanisms for internal combustion engines.
According to an aspect of the invention, a variable valve actuation mechanism is provided for an internal combustion engine comprising at least one valve for control of gas admission to a cylinder of the engine and/or gas exhaust from the cylinder, comprising
two concentrically arranged camshafts,
a cam set comprising two cams, each fixed to a respective of the camshafts, whereby the camshafts are arranged to be turned in relation to each other, so as to change the combined profile of the cams, and
a cam follower adapted to follow the combined profile of the cams and to actuate at least one of the at least one valve in dependence on the combined profile of the cams,
wherein the cam follower comprises two rollers, each roller being adapted to follow a respective of the cams.
It is understood that, the cam follower is adapted to be in contact with the cams, and to thereby follow the combined profile of the cams so as to actuate at least one of the valves in dependence on the combined profile of the cams. The rollers are adapted to provide the contact of the cam follower with the cams. The cams may be are arranged to be moved in relation to each other by turning of one of the camshafts in relation to the other, so as to change the combined profile of the cams.
Since the cam follower comprises two rollers, each roller being adapted to follow a respective of the cams, the risk of contact of a roller with an edge of any of the cams is greatly reduced. This in turn provides for significantly reducing wear in the variable valve actuation mechanism. More specifically, with the double roller solution, it is possible to avoid a situation where a roller surface bridges the two cams, and is thereby exposed to potential contact with the cam edges. Further, as also exemplified below with reference to FIG. 7, the double roller solution provides for avoiding skidding of a roller surface against a cam surface. Without the two roller solution, such skidding may occur, e.g. when the cam follower transits from one of the cams to the other one, and due to local differences in the inclination or declination of the cams, the rotational speeds to which the cams urges a single roller will be different. Two rollers will solve this problem by allowing individual adaption of the rotational speed to the respective cam. Thus, the invention provides for reducing wear caused by skidding as well as edge contact.
Preferably, at least one of the rollers presents a contact surface having a crowning profile. As also explained below, this increases tolerances to misalignment in a manufacturing process as well as misalignment due to operating loads, and further reduces the risk of edge contact between a roller and a cam. The crowning may provide, in the axial direction of the roller, a variation of 0.005-0.050 mm, preferably 0.010-0.030 mm, of the radial position of the contact surface.
At least one of the rollers may present a contact surface having a crowning profile with a crowning shape of a logarithmic function, or a function in the form of Y(X)=AX{circumflex over ( )}B where A and B are real numbers and B is greater than 2. At least one of the rollers may present a contact surface having a crowning profile providing a part-circular outer surface for contacting its associated cam.
Preferably, at least one of the rollers presents a contact surface having a smaller extension in an axial direction than its associated cam. Thereby, it can be made sure that an angular misalignment between the rollers and the cams does not lead to any contact between a cam edge and a roller. If in addition the rollers are crowned, contacts between the cams and the rollers, without any edge contact, will be secured.
Preferably, the axial freedom of movement of the roller is shorter than the difference between the axial extensions of the contact surface of the roller and its associated cam. Thereby, possible axial movements of the roller may be kept within the axial extension of the cam, which in turn eliminates any risk of contact of the roller with one of the cam edges. This in turn reduces the risk of excessive wear. The allowed axial movement of each rollers might be 1.0-10.0%, preferably 1.7-5.0%, of the axial extension (width) of the roller. In some embodiments, each roller is fixed in the axial direction of the roller, in relation to the respective cam which the respective roller is adapted to follow.
Preferably, the rollers are fixed concentrically in relation to each other. Preferably, the cam follower comprises two support arms and wherein the rollers are both mounted between the two support arms. Preferably, the cam follower comprises a shaft, which is supported at each end in one of the two support arms and wherein the rollers are concentrically arranged on the shaft. Preferably, the cam follower comprises a shaft, the rollers being concentrically arranged on the shaft via respective sliding bearings. Preferably, the shaft is provided with a friction reducing layer, for example a PVD (physical vapour deposition) coating. The shaft is advantageously made of steel; alternatively the shaft might be made in any suitable alternative material, such as a bronze alloy. The rollers might be made of steel, but any suitable material alternative is possible.
Preferably, each roller presents a heel at each end of its axial extension. Each heel might be provided as an axial protrusion presenting a flat surface oriented in a plane with a normal which is parallel to the axial direction of the respective roller.
Preferably, the rollers are adapted to turn independently of one another. Preferably, the rollers have substantially the same extension in an axial direction and/or radial direction. The rollers may have different extensions in the axial direction; this may provide benefits where the loadings on the rollers are different, and there is a lack of space around the rollers.
According to another aspect of the present invention, a variable valve actuation mechanism is provided for an internal combustion engine comprising at least one valve for control of gas admission to a cylinder of the engine and/or gas exhaust from the cylinder, comprising
two concentrically arranged camshafts,
a cam set comprising two cams, each fixed to a respective of the camshafts, whereby the camshafts are arranged to be turned in relation to each other, so as to change the combined profile of the cams, and
a cam follower adapted to follow the combined profile of the cams and to actuate at least one of the at least one valve in dependence on the combined profile of the cams,
wherein the cam follower comprises a roller presenting, in a cross-section coinciding with a rotational axis of the roller, two protuberances being adapted to follow a respective of the cams, the protuberances being separated by a concavity.
According to another aspect of the invention, an internal combustion engine is provided comprising a variable valve actuation mechanism according to any of the embodiments described or claimed herein, and by a vehicle comprising such an engine.